Document EP.O.471.136 in particular already discloses a device for extracting a bulk product stored in a silo comprising a central evacuation opening. This extraction device is constituted by a vertical central pole driven in rotation on itself, a support inclinable with respect to the central pole, and an Archimedean screw mounted on the support and driven in rotation on itself. The base of the support is fixed to the vertical central pole via a pivot connection of horizontal axis, and adjustment of the inclination of the support and consequently of the Archimedean screw, is effected with the aid of a suspension cable of which one end is fixed to the inclinable support, and of which the other end is wound around a winch fixed to the top of the central pole, in the vicinity of the top of the silo.
Prior to the silo being filled, the support and the Archimedean screw are maintained in equilibrium, via the cable, substantially along the central pole. Then, in order to empty the silo, the Archimedean screw is firstly rotated on itself, which has for its to disengage it, then the central pole is rotated on itself, which provokes rotation of the support and of the Archimedean screw about the central axis of the silo. Finally, the angle of inclination of the support and of the Archimedean screw with respect to the central pole is progressively increased by unwinding the suspension cable. Consequently, the Archimedean screw works on surface of the stored product, in a progressively larger upturned cone as the angle of inclination of the Archimedean screw with respect to the central pole increases, and pushes the product towards the central evacuation opening. As the Archimedean screw does not work in the mass of the product, except at start-up, the installed power necessary for this extraction device to operate is a function in particular of the dimension and mass of the Archimedean screw and of the support, and depends only little on the mass of the product to be extracted.
However, this extraction device presents the following drawbacks: Prior to the silo being filled, the support and the Archimedean screw, due to their articulation with respect to the central pole, cannot be positioned in a strictly vertical plane. Consequently, they must forcibly be maintained in equilibrium in their initial position before filling, via the suspension cable, order to avoid their pivoting in a vertical plane under the action of their own weight. The silo in which this extraction device is mounted, is filled by introducing by gravity the bulk product to be stored, via an opening made at the top of the silo. The bulk product thus introduced exerts a high pressure on the support and the Archimedean screw, which provokes a considerable increase in the tension of the suspension cable and consequently in the tension that the central pole must support addition, when emptying of the silo is terminated, order to return the support and Archimedean screw into initial position before filling, it is necessary to pivot them in a vertical plane through an angle of about 90.degree., by winding the suspension cable. In that case, the central pole must also be able to withstand the tension exerted by this suspension cable, whatever the angle of inclination of the support and of the Archimedean screw with respect to the central pole, this tension increasing with the angle of inclination. In conclusion, the central pole must be sufficiently rigid and be sufficiently anchored vertically in order to withstand the tension of the cable, this increasing with the storage capacity of the silo and the weight of the Archimedean screw and of the support.
In the extraction device of document EP.O.471.136, it is necessary, at the beginning of extraction, for the extraction screw to be able to be rotated about the vertical axis of the central pole, that this screw be disengaged from the bulk product in which it is embedded. To that end, it is necessary, on the one hand, that this screw be of the Archimedean type, and, on the other hand, that it may be rotated on itself, so as to evacuate the bulk product towards the central opening of the storage area. Such rotation of the extraction screw on itself, when it is embedded in the mass, requires a considerable installed power. This installed power must be all the greater as the bulk product may form conglomerates which oppose this rotation.
In practice, it has proved that the use of an extraction device such as the one described in document EP.O.471.136, with an Archimedean screw whose length exceeds 15 meters, is very difficult due, on the one hand, to the too high tensions to which the suspension cable and the central pole are subjected and, on the other hand, the installed power necessary for start-up of the extraction screw. This extraction device is therefore unsuitable for emptying silos having a large storage capacity anon in particular silos of circular base whose ground diameter exceeds 30 meters.
It is an object of the present invention to propose a device for extracting a bulk product, of the bulk sugar, clinker, fly ash . . . type, stored over a storage area comprising at least one evacuation opening, and which overcomes the drawbacks of the extraction device set forth hereinabove.